1. Field of the Invention
The present invention relates to the field of disk drives. In particular, the present invention relates to a disk drive having a read/write element having reduced off-track motion.
2. Description of the Related Art
FIG. 1 shows an exemplary high-RPM hard disk drive (HDD) 100 having a magnetic read/write head (or a recording slider) 101 that includes, for example, a tunnel-valve read sensor, that is positioned over a selected track on a magnetic disk 102 using, for example, a two-stage servo system for reading data stored on disk 102. The two-stage servo system includes a voice-coil motor (VCM) 104 for coarse positioning a read/write head suspension 105 and may include a microactuator, or micropositioner, for fine positioning read/write head 101 over the selected track. As used herein, a microactuator (or a micropositioner) is a small actuator that is placed between a suspension and a slider, and moves the slider relative to the suspension.
There are several types of microactuators that are conventionally used. One type is referred to as a moving-slider-type microactuator. Another type is referred to as a moving-head-type microactuator. The basic structure for both of these types of microactuators is the same and includes a structure that allows relative motion like a spring, and a mechanism that generates a driving force. One side of the microactuator is attached to a suspension load beam, which is a relatively solid structure. The other side of the microactuator is attached to a structure on which a read/write element is located, such as a slider.
FIG. 2 depicts block diagram model for a moving-slider-type microactuator arrangement 200. A microactuator, which is modeled as a spring 201 in FIG. 2, is placed between a suspension 202 and a slider 203. Suspension 202 is relatively rigid in the off-track direction, while microactuator 201 is relatively flexible in the off-track direction. Consequently, slider 203 is also relatively flexible in the off-track direction, as depicted by arrow 204. Exemplary moving-slider-type microactuators are disclosed by U.S. Pat. No. 6,078,473 to Crane et al. and U.S. Pat. No. 6,476,538 to Takeuchi et al.
FIG. 3 depicts a block diagram model for a moving-head-type microactuator arrangement 300. For a moving-head-type microactuator, a microactuator 301 is placed between a slider body 302 and a read/write element 303. Similar to a moving-slider-type microactuator, slider body 302 is rigid in the off-track direction, while microactuator 301 is relatively flexible in the off-track direction so read/write element 303 is also relatively flexible in the tracking direction, as depicted by arrow 304
Accordingly, a common problem with both a moving-slider-type and a moving-head-type microactuator is an increased level of off-track positional disturbances that are caused by the combination of the flexibility of the spring mechanism of the microactuator and a force applied to the moving part (e.g., the airflow). In a typical HDD, the slider and R/W element are exposed to very fast and usually turbulent airflow that is caused by the spinning disk. For non-microactuator HDDs, the slider and R/W element are respectively fixed to the suspension and to the slider body and are rigid in an off-track direction. Thus, airflow force does not cause off-track positional disturbance problems. When, however, a microactuator is flexible in an off-track direction, such as a moving-slider-type or a moving-head type microactuator, force generated by air-flow can easily produce off-track positional disturbances.
FIG. 4 depicts a more detailed block diagram model for a suspension load beam having a microactuator. As shown in FIG. 4, the spring mechanism of the VCM is modeled by a spring having a spring constant k1 and the spring mechanism of a microactuator is modeled as a spring having a spring constant k2. The damping factors for the VCM and the microactuator are respectively modeled by damping factors c1 and c2. Rotational inertias Iz1 and Iz2 represent the respective rotational inertias associated with the VCM and microactuator.
FIG. 5 shows a graph of calculated off-track positional disturbances for an exemplary conventional non-microactuator suspension load beam and for an exemplary conventional microactuator HDD using the model of FIG. 4. The following parameters are assumed for the model of FIG. 4: The force input is from an air-flow that creates a torque disturbance having an amplitude of 2.5×10−7 Nm that is applied to the microactuator. The inertia of the VCM is 4×10−6 kg*M2. The resonant frequency of VMC is 70 Hz. The quality factor of VCM main resonance is 5. The length of VMC is 50 mm. The microactuator type is a moving-slider-type microactuator, which moves the slider in rotational direction. The rotational inertia of the moving part of the microactuator including the slider is 5.7×10−13 kg*m2. The main resonant frequency of the microactuator is 2 kHz, and the quality factor of the microactuator is 30.
The abscissa of FIG. 5 is frequency in Hz, and the ordinate of FIG. 5 is disturbance measured in meters (m). Curve 501 is the disturbance experienced by a non-micro VCM. Curve 502 is the disturbance experienced by a microactuator and VCM combination. The graph of FIG. 5 shows two problems that are experienced by a microactuator/VCM combination. First, a microactuator/VCM combination exhibits a resonance at 2 kHz, resulting in a large disturbance peak at 2 kHz. Second, above about 100 Hz, a microactuator/VCM combination exhibits a higher level of disturbance than a non-micro VCM. Further, the level of disturbance experienced by a microactuator/VCM combination above the resonant frequency of the microactuator is almost three orders of magnitude greater that the level of disturbance experienced by a non-microactuator VCM HDD.
FIG. 6 is a graph showing measured positional disturbance data for a non-microactuator HDD and an HDD having a microactuator/VCM combination. The microactuator used for both the microactuator/VCM combination is a moving-slider-type microactuator that moves the slider in a rotational direction. The parameters associated with the non-microactuator HDD and the HDD having a microactuator/VCM combination of FIG. 6 roughly correspond to the parameters for the devices of FIG. 5. Curve 601 (blue) is the measured positional disturbance for a non-microactuator VCM HDD. Curve 602 (red) is the measured positional disturbance for a microactuator/VCM combination HDD. Curves 601 and 602 exhibit tendencies that are similar to the corresponding curves shown in FIG. 5.
What is needed is a technique for reducing positional disturbances experienced by a moving-slider-type microactuator and a moving-head-type microactuator.